In many of the applications in which they may be used, analog filters require repeated tuning while in operation in the field. Such applications include cellular telephones, high fidelity audio equipment, multi-media computers, and sensors. In other applications, analog filters may require a one-time tuning after fabrication is completed, which tuning may be performed by trimming immediately after fabrication or at a later time by an original equipment manufacturer (OEM) prior at come point prior to final packaging of the equipment. Particularly, it is difficult to precisely match resistors and capacitors during fabrication of integrated circuit chips. Variations from chip to chip of resistance and capacitance values can range as high as 20 percent. Since the frequency characteristics of a filter are primarily dependent upon the values of the resistors and capacitors comprising the filter, the frequency characteristics of a filter at the end of fabrication may not meet the tolerance requirements for the given application. Accordingly, it is frequently necessary to fine tune filters after fabrication is completed.
One method for implementing filters so that tuning can be avoided is to use switched capacitor filters which emulate a continuous time filter (i.e., an analog filter). These do not require tuning as their performance is dependent of the ratio of capacitors and the clock frequency. However, switched capacitor filters require clock rates considerably higher than the operating band width of the filter (e.g., anywhere from about 5 to 100 times faster). Thus, they require operational amplifiers with a commensurately wider band width and, hence, larger power consumption and area requirements. Also, the transistors used in the switches do not operate well at low supply voltages because the switches have too much resistance associated with them. Thus, the internal nodes do not settle and filter response suffers.
With respect to portable electronic equipment, such as cellular telephones, pagers, lap top computers, etc., there is a drive towards using lower voltages in order to conserve power and, thus, increase battery charge life before a recharge or battery replacement is necessary. However, as the operational voltages for the components get lower and lower, it sometimes causes problems in operation because transistors which require a minimum voltage to turn on (or off) are not receiving the voltages necessary to turn them fully on (or off). This problem may result in even further variation in frequency response of filters.
While, in some applications, a one-time post-fabrication tuning may be adequate, component characteristics also commonly drift over time during operation causing further changes to the frequency response characteristics of the filter. Accordingly, in many applications, it is desirable to fine tune analog filters repeatedly over time while they are in use in the field. There are several known ways of tuning filters after fabrication and/or repeatedly during field operation. One well-known scheme involves trimming of the resistors and/or capacitors forming the filter after fabrication, but before installation in its application environment. Trimming, however, is disadvantageous in that it increases manufacturing cost and time and is a one-time tuning that does not allow for further tuning in the field as the circuit components age.
Other known schemes involve the use of multiple, identical analog filters. In one such multiple identical filter implementation, a first filter is in operation filtering the actual data, while a second, identically fabricated filter is being tuned. The second filter is tuned by placing a known test pattern at its input and reading the filter output in response thereto to determine the filter's frequency characteristics. The filter under test is tuned to cause its frequency characteristics to more closely match the desired characteristics. Then the two filters are switched so that the second filter that was just tuned now operates on the actual data, while the first filter receives the test patterns and is tuned. The alternate functions of the two filters are switched at regular intervals in order to assure that the filter which is operating on the actual data is regularly corrected as necessary.
In another dual matched analog filter implementation known as a master/slave implementation, one filter, the slave, is always utilized in connection with the actual data and the other filter, the master, is constantly undergoing testing for tuning purposes. Both the master and slave filters will be tuned identically and simultaneously based on the test results for the master filter. One disadvantage of multiple filter tuning implementations is that they double the number of filters and, hence, increase area and power requirements. The master/slave scheme is further disadvantageous in that the master and slave filters may not be perfectly matched. Thus, the tuning which is effective for appropriately setting the characteristics of the master filter may not place the slave filter (the one that is operating on the actual data) to the correct characteristics.
Co-pending U.S. patent application Ser. No. 08/502,591, assigned to the same assignee as the present application and incorporated herein by reference, discloses another method and apparatus for frequency tuning of continuous-time or analog filters. Particularly, a switch is provided at the input of an analog filter in order to allow the input to be alternately connected to either the actual input signal (normal mode operation) or a test signal (test mode operation). When in the test mode, the output of the filter is coupled to a processor which analyzes the output in response to the input test signal to determine the frequency characteristics of the filter. The processor then controls the filter to tune it responsive to the difference between the expected filter output responsive to the test signal and the actual output of the filter.
Particularly, for a low pass filter, the test signal comprises a DC signal followed by a sinusoidal AC signal at the desired cut-off frequency of the filter, i.e., the -3 dB (decibel) point. During test, first the DC signal is applied and the filter output stored in memory and then the AC signal is applied and the filter output stored in memory. The two values are compared to determine if the AC signal is three decibels lower than the DC signal and, if not, the filter is tuned accordingly.